Method and improved furnance for reducing emissions of nitrogen oxides

ABSTRACT

A method and improved furnace for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors and a plurality of spaced apart over-fire air injectors positioned above the primary fuel injectors are disclosed. Injection of over-fire air produces zones of cooler combustion gasses containing over-fire air that separate zones of hot combustion gasses containing nitrogen oxides. Reburn fuel injectors inject a reburn fuel into the zones of hot combustion making the effluent combustion gases containing nitrogen oxides partially or totally fuel-rich in order to further reduce nitric oxide.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 62/197,731 filed Jul. 28, 2015.

FIELD OF INVENTION

The invention relates to methods for reducing nitrogen oxide emissionsfrom coal-fired, oil-fired and gas-fired furnaces.

BACKGROUND OF THE INVENTION

Nitric oxide, formed in combustion of fuels in an enclosed furnace, maybe reduced by reburning a second fuel in the effluent combustion fluegas. Conventional reburn practice in the 1980's was that a cross-sectionof the furnace be made fuel-rich by injection of a reburn fuel, followedby the injection and mixing of completion air, in the loweredtemperature region of the upper furnace. U.S. Pat. No. 5,915,310, issuedon Jun. 29, 1999, taught that the reburn fuel stream could be injectedinto the fuel lean effluent flue gas to form individual pockets oreddies of fuel-richness which would reduce nitric oxide before mixingand dissipating into the overall fuel lean flue gas, without the needfor completion air. Both of these techniques require that completion ofreburn combustion or dissipation of fuel-rich eddies be completed withinthe limited volume of the upper burn-out region of the furnace.

Many furnaces have ports above the burners which inject air, calledover-fire air, into the furnace. However, with the use of over-fire airas the prevalent method of nitric oxide reduction in operating furnaces,the lower primary combustion zone of the furnace is operated withlowered excess air (thus reducing nitric oxide formation in that hightemperature region) and the staged over-fire air is injected into theupper furnace in order to complete combustion. The result is that withfurnaces using over-fire air, the limited volume of the upper furnace isno longer available for the application of conventional or even FuelLean Reburn.

Reburn technology involves injecting a fuel into previously combustedflue gas, after that gas has cooled, for the purpose of reducingunwanted pollution species, in particular nitric oxide (NOx). Naturalgas is easily injected and combusted as the reburn fuel of choice atthis lower temperature, but other fuels may be used as the reburn fuel.However, application of over-fire air technologies for NOx reductionseemingly has left no room for the application of reburn injection andcombustion completion.

Wherever the reburn zone can be made fuel-rich, any nitric oxide (NO)previously formed in the primary combustion, high temperature adiabaticzone, will act as a strong oxidizer upon mixing into fuel-richness toform C—N and/or H—N or even N₂ type species. Nitric oxide is reduced inthis fuel-rich reburn zone but it may reform when the fuel-rich zonemixes with completion air in the upper furnace in order to completecombustion. The final nitric oxide emission and efficiency of thisreburn teaching depends upon the relatively lower temperature of theupper furnace volume where this completion of combustion takes place.

Application of In-furnace NOx reduction patented by Mitsubishi in 1981and generally all reburn technologies have been limited by the fixedvolume available and high furnace exit temperature in the upper furnaceof practical boiler designs. If too much reburn fuel is injected at thelowered temperature of the upper furnace, for a given furnace volume,then the completion of combustion will not proceed and the concentrationof CO and unburnt hydrocarbons will increase beyond acceptable emissionand safety limits. Similarly if the furnace exit temperature is highbecause of limited furnace volume then the final nitric oxide will behigher upon combustion completion.

The application of NOx Ports or over-fire air allow the lower furnace,primary flame adiabatic combustion zone to be operated with low excessair and even locally fuel-rich firing, thereby lowering or eliminatingthe formation of an oxidizer. The over-fire air is then injected intothe upper furnace in order to complete combustion at a much loweredtemperature, thereby reducing the kinetics of nitric oxide formation,which are very dependent upon both temperature and excess oxygen.

The prevalent application of over-fire air technology is depicted inFIGS. 1 and 2 as front and side views of a furnace, FIG. 1a , withoutover-fire air and a furnace, FIG. 1b , with over-fire air. Anywhere fromone to multiple rows and columns of burners 4 may be configured on thefront wall of the furnace and also often may be configured on the rearwall or corners of the furnace. The lower, nominally ⅓ region 6 of thefurnace volume contains the burners 4 which inject a combustibleair/fuel mixture and constitute the primary flame region of the furnace.The region is shaded in FIG. 1a and FIG. 1b . The addition of over-fireair ports 8 are depicted as squares directly above the columns ofburners 4 in FIG. 1 b.

Normally the flames propagate from the lower burner region into theupper ⅔ of the furnace as the fuel burns-out to complete combustion, asdepicted in FIG. 2a . And considering normal design, this volume ispredicated on the design of the boiler and the heat balance design ofthe power generation system. However, the application of over-fire airtechnology, for NOx reduction, requires all of the upper furnace to beused to complete the combustion, due to the lack of sufficient excessair in lower primary flame region of the furnace. The upper ⅔ and morecritically the upper ⅓ of the furnace volume now is required for thecomplete mixing of the over-fire air to provide adequate excess airbefore furnace exit, as shown in FIG. 2b . Therefore, there seeminglyappears to be no volume available for the injection of reburn fuel andits own mixing and combustion.

Consequently, there remains a need for a furnace design and method whichreduces nitrogen oxide emissions from coal fired and gas fired furnaces.Preferably this new furnace design can be created by retrofitting ormodifying existing furnaces. More preferably the new design and methodshould be able to be implemented without any major modifications ofexisting furnaces.

SUMMARY OF THE INVENTION

We provide a method and improved furnace for reducing nitrogen oxideemissions from a furnace having primary fuel injectors and spaced apartover-fire air injectors positioned above the primary fuel injectors.Injection of over-fire air produces zones of cooler combustion gassescontaining over-fire air that separate zones of hot combustion gassescontaining nitrogen oxides. The improved furnace has reburn fuelinjectors that inject a reburn fuel into the zones of hot combustiongases making the effluent combustion gases containing nitrogen oxidespartially or totally fuel-rich in order to further reduce nitric oxide.

Our process is such that the reburn fuel reacts, within a purposefullytargeted fuel-rich zone, with the nitric oxide (NO) formed in the lowerfurnace. Thus reducing nitric oxide previously formed. Also, the higherhydrogen content of the reburn fuel, such as natural gas, reacting withnitric oxide in this, now fuel-rich zone, forms not only CO, H₂O, andN₂; but also amine type N—H species or radicals which in turn enhancethe nitric oxide reduction. At the same time, the higher hydrogencontent of the reburn fuel improves the final burn-out of the carbon inthe primary fuel.

Our method of application involves identifying the zones of hot chimneyflow of the high temperature flue gas around the colder and denserover-fire air injection columns. Then the method involves penetratingbetween existing boiler tubes of the furnace proper to inject the reburnfuel into the high temperature flue gas, which contains nitric oxideformed in the lower furnace. By our method we establish multipleisolated zones of low excess air or fuel-rich, off-stoichiometriccombustion of the reburn fuel which reduces nitric oxide, enhancescombustion of low hydrogen carbon fuels and mixes or dissipates into thecurrently operating over-fire air system.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a is a diagram of a wall in a conventional furnace which has noover-fire air ports.

FIG. 1b is a diagram of a wall in a conventional furnace which hasover-fire air ports.

FIG. 2a is a side-view diagram showing the flow of combustion gases in aconventional furnace which has no over-fire air ports.

FIG. 2b is a side-view diagram showing the flow of combustion gases in aconventional furnace which has over-fire air ports.

FIG. 3 is a diagram showing zones in the flow of combustion gases thatwe have observed in a furnace which has over-fire air ports.

FIG. 4 is a diagram similar to FIG. 3 showing zones in the flow ofcombustion gases in a present preferred embodiment of our furnace.

FIG. 5a is a diagram of a present preferred reburn fuel injector thatcan be used in the furnace shown in FIG. 4.

FIG. 5b is a diagram of a portion of furnace wall having a slot in thewebbing between boiler tubes, into which slot the reburn fuel injectormay be placed.

FIG. 6 is a diagram showing plume penetration of the reburn fuel intothe combustion gas flowing through the furnace shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It inherently takes the total volume of the upper furnace to fully mixthe over-fire air with the combustion effluent from the lower furnace;other-wise, the lower ⅓-primary flame combustion zone might be made evenmore fuel-rich with increased over-fire air and improved NOx reductions.

Considering the effect of high temperature upon increasing nitric oxidekinetics, over-fire air injection and mixing is generally accomplishedwithin the colder, upper two-thirds of the furnace. This is accomplishedby diverting a part of the normally used wind box combustion air. Wehave discovered that as the over-fire air mixing takes place, theredevelop upward flowing parallel zones of buoyant effluent combustiongases which are in the process of mixing but are not yet mixed. The windbox combustion air is preheated to between 500° F. and 900° F., but itis relatively cold in comparison to the combustion effluent which may behotter than 3000° F. Thus, the injected over-fire air is two or threetimes more dense than the uprising combustion effluent flue gas. Becauseof this large density difference the hot and buoyant combustion effluentchimneys around the colder over-fire air and the over-fire air mixesvery poorly. This parallel flow of over-fire air is depicted in FIG. 3,in an actual 460MW Utility Boiler application.

These upward flow, buoyant columns of hot gases have been visuallyobserved, empirically measured with a matrix of CO, O₂ and NOmeasurements and replicated through Computer Furnace Models. Thesecolumns of hot gases or spirals, in the case of tangential-firedfurnaces, contain the nitric oxide formed in the lower primarycombustion region of the furnace.

We identify and target these parallel columns or spirals of combustioneffluent, containing nitric oxide for injection of a reburn fuel. Wecall this injection Zonal Gas Reburn injection. Zonal Gas Reburn mayalso be called ZGR or Zonal Off-Stoichiometric Gas Reburn or ZOGR.

In a present preferred embodiment shown in FIGS. 4 and 5 b, multipleslot injectors 21 are inserted between the boiler tubes 22 of an actual460 MW Utility Boiler 20. These zonal gas reburn injectors are targetedto inject the reburn gas into the combustion effluent containing nitricoxide formed in the primary flame region 12 of the furnace.

In the case of this 460MW Boiler shown in FIG. 3, the reburn fuelinjectors 21 are placed in a region between the centerlines 14 ofadjacent over-fire air injectors 8 and above those injectors. The reburnfuel injectors in the furnace shown in FIG. 4 are 14 feet above theover-fire air injectors and over-fire air injectors are 13 feet abovethe top row of burners. In general the reburn fuel injectors may beplaced above or below the over-fire air injectors but in position to mixwith the trajectory of the effluent combustion gases, containing thenitric oxide to be reduced. This applies to all type of furnaces whetherface-fired, opposed-fired as here, tangentially-fired, turbo-fired oreven grate-fired; in every case with over-fire air, the targeted zonewill exist before and as the over-fire air mixes. The resulting zonalreburn zone, regardless of furnace type, must extend to the coldestpossible region of the upper furnace in order to take full advantage oflowered temperature nitric oxide equilibration, as this reburn zonemixes to complete combustion with the over-fire air.

The gas reburn fuel injectors 21 may be any type orifice, round, square,oblong, etc., so long as it may fit through a penetration into thefurnace or even through the air duct around the primary burners and theyinject the gas reburn fuel into the targeted nitric oxide containingeffluent.

We prefer to provide multiple reburn fuel slot injectors to allow fora-priori targeting of the high NOx zones with multiple injectors. Weprefer to provide reburn fuel injectors which fit between tubes 22without requiring expensive tube modifications as shown in FIGS. 5a and5b . The reburn fuel injectors 21 may be horizontally adjustable as theyfit through the slots 24 in the webbing 26 between the boiler tubes, asis the one installed as shown in FIG. 5 b.

Vertical tilt or up/down adjustment of the slot injector mechanismsshown in FIG. 5a allows for empirical experimentation and adjustmentsfor maximizing nitric oxide reduction and maintaining combustioncompletion with minimum CO emissions.

Alternatively, the injectors could be positioned to penetrate the boilerthrough existing openings. Such openings could be openings for theprimary burner air duct.

The use of high hydrogen containing natural gas or other high hydrogengas, such as coal or syn-gas improves the burn-out of lower hydrogenprimary fuels such as coal or heavy oils. With natural gas reburninjection the moisture content of the effluent combustion gas goes from6% in the primary furnace to nominally 12% in the Zonal Gas Reburn zoneof our process. This increased hydrogen content greatly enhances carbonburn-out; and our adjustable and parallel flow reburn injection methodallows the process to take place in the most favorable temperatureregion of the furnace which will allow for both combustion completionand lowered temperature nitric oxide equilibration.

In the case of the 460MW boiler shown in FIG. 3, two rows of seven ZonalGas Reburn injectors were installed on the front wall and two rows ofseven Zonal Gas Reburn injectors were also installed on the rear wall toallow flexibility when operated at reduced load and with differentcombinations of coal mills in service. Also one row of four injectorswere installed on the boiler sides to allow reduced load following andemission minimization. The measured penetration produced by the gasinjected through these ZGR injectors was twenty-eight feet. This isshown proportionately in FIG. 6 for this seventy-eight foot high furnacevolume boiler.

In the demonstration case shown in FIG. 6, the ZGR injector is pointeddownward at 20 degrees and the ZGR injection plume mixes for 28 feetbefore losing its identity within the uprising combustion effluent. Eachgas plume spread to 6 feet in diameter at 28 feet penetration and movedupward with the combustion gases at a velocity of 10 feet to 30 feet perhalf second. At 28 feet the plume eddies could still be seen and arefuel rich. Six feet of the plume had mixed with approximately 60% of thecombustion gases such that the plume contained 9 percent fuel (0.6×15%excess air=9% fuel). Then the plume mixed out to form final CO.

In our method a gaseous fuel can be injected into a furnace whichalready is using over-fire air for nitric oxide reduction and throughour process of targeted Zonal Gas Reburn further nitric oxide reductionscan be achieved.

The gaseous fuel injection can be selectively targeted to zones ofbuoyant parallel or spiral flow combustion effluent, which chimneyaround the colder over-fire air flow, to form multiple reburn zones. Ourtargeted reburn zones rise with the buoyant effluent, which containsnitric oxide formed in the lower furnace, and our fuel-rich Zonal GasReburn reduces this nitric oxide. Also, the hydrogen content of thereburn fuel improves the carbon burn-out of solid or liquid primaryfuel; while the targeted reburn zone dissipates in the upper furnace,with the over-fire air serving as reburn completion air. Our targetedZonal Reburn Injection reduces both nitric oxide, CO and carbonemissions.

Buoyant, vertical or spiral (encountered with tangential firing) zonesof effluent, containing nitric oxide from the lower primary fuelcombustion region of the furnace can be spatially identified and zonalgas reburn injection can be used to form upward flowing fuel-rich reburnzones which reduce the nitric oxide and then mix with the over-fire airto complete combustion. These zones can be identified by observation ofa furnace in which temperatures of the combustion gases are measured atselected locations or by computer modeling. The computer modeling mayinclude flow modeling of high velocity injectors designed to form nitricoxide reburn zones.

Our process spatially targets multiple zones of a furnace to reducenitric oxide emissions and at the same time improve combustion andburn-out.

Our process is compatible with the implementation of over-fire air. OurZonal Gas Reburn in fact uses the already implemented over-fire air ascompletion air to complete the combustion of the Zonal Reburn Fuel.

Reburn gas or higher hydrogen reburn fuel can be injected through boilerwall penetrations or even through the primary burner air supply totarget the primary combustion effluent and reduce nitric oxide. Suchreburn fuel injectors are purposely targeted to cause the combustioneffluent to become fuel-rich thereby reducing its nitric oxideconcentration. These injectors can be designed to maximize the reductionof nitric oxide within the targeted effluent. Their design may involveany orifice suitable to provide the design penetration into the targetednitric oxide containing effluent. The orifice can be round, square oroblong orientation and aspect to provide the design penetration into thetargeted nitric oxide containing effluent.

Slots can be installed in the webbing between boiler tubes, withoutmodification of these tubes for using our method. Through use of theseslots significant gas reburn fuel can be injected into targeted furnaceeffluent flow zones. And further these targeted zones of low excess aircombustion effluent are thereby made fuel-rich, so that resultingoff-stoichiometric combustion of the reburn gas will reduce nitric oxidein this effluent.

Slots can be cut between tubes allow the insertion of practicalinjection nozzles for the purpose of controlled injection and mixing ofreburn fuel into targeted effluent arising from the lower furnace.

Practical injection nozzle assemblies can be fastened to the boiler wallso as to fit through the slots in the webbing. These injection nozzleassemblies allow vertical directional adjustment of injection flow, andthe tolerance space around the inserted nozzles allows for their aircooling.

Our reburn fuel injection assemblies may be installed to providemultiple Zonal Gas Reburn injection points without expensive highpressure boiler tube modifications.

Multiple reburn fuel injection nozzles can have a high vertical aspectratio of vertical height to horizontal width from 1.0 to 100 in thedirection of the upward flowing effluent, thus providing more targetedgas flow in the upward direction of the effluent and less horizontalspreading into the over-fire air (where there is no nitric oxide).

Slotted reburn fuel injectors can be positioned to target observed zonesof primary combustion effluent, before the effluent mixes with over-fireair, for any type furnace such as, but not limited to: face-fired,opposed-fired, tangential-fired or turbo-fired furnace.

Although we have shown and described certain present preferredembodiments of our furnace and method for reducing nitrogen oxideemissions it should be distinctly understood that our invention is notso limited and may be variously embodied within the scope of thefollowing claims.

We claim:
 1. An improved furnace of the type having a plurality ofprimary fuel injectors on a wall of the furnace and a plurality ofspaced apart over-fire air injectors on the wall of the furnace andpositioned above the primary fuel injectors such that centerlinesthrough each a pair of adjacent over-fire air injectors define a regionon the wall of the furnace that extends upward and downward on the wallof the furnace between that pair of adjacent over-fire air injectors,wherein the improvement comprises a plurality of reburn fuel injectors,each reburn fuel injector located within one of the regions.
 2. Theimproved furnace of claim 1 wherein the reburn fuel injectors are slotinjectors.
 3. The improved furnace of claim 1 wherein the reburn fuelinjectors are capable of being tilted to direct the flow of reburn fuelin a selected upward direction or a selected downward direction.
 4. Theimproved furnace of claim 1 wherein the reburn fuel injectors arecapable of being turned left or turned right to direct the reburn fuelin one of several selected directions.
 5. The improved furnace of claim1 wherein the furnace is comprised of a series of substantially parallelboiler tubes and webbing extending between each pair of adjacent boilertubes and the reburn fuel injectors are in the webbing.
 6. The improvedfurnace of claim 1 wherein the reburn fuel injectors are positionedabove the over-fire air injectors.
 7. The improved furnace of claim 6wherein the reburn fuel injectors are positioned about fourteen feetabove the over-fire air injectors.
 8. The improved furnace of claim 1wherein the furnace is a face-fired furnace, an opposed-fired furnace, atangentially-fired furnace, a turbo-fired furnace or grate-firedfurnace.
 9. The improved furnace of claim 1 wherein the reburn fuelinjectors have an orifice that is round, square or oblong.
 10. Theimproved furnace of claim 1 wherein the injectors penetrate the boilerthrough existing openings.
 11. The improved furnace of claim 10 whereinthe furnace has a primary burner air duct and at least one of theexisting openings are the primary burner air duct.
 12. A method forreducing nitrogen oxide emissions from a furnace having a plurality ofprimary fuel injectors on a wall of the furnace and a plurality ofspaced apart over-fire air injectors on the wall of the furnace andpositioned above the primary fuel injectors wherein during operation ofthe furnace combustion gasses will flow from the ignition zone adjacentthe fuel injectors toward and past the over-fire air injectors and theinjection of over-fire air creates zones of hot combustion gasescontaining nitrogen oxides, the zones separated by zones of coolercombustion gases containing over-fire air, the method comprisinginjecting a reburn fuel into at least one of the zones of hot combustiongases.
 13. The method of claim 12 wherein the reburn fuel is injected ata downward or upward angle into the at least one of the zones of hotcombustion gasses.
 14. The method of claim 12 wherein the reburn fuel isa high hydrogen content gas.
 15. The method of claim 14 wherein the highhydrogen content gas is natural gas or coal syn-gas.
 16. The method ofclaim 12 also comprising identifying the zones of hot combustion gases.17. The method of claim 16 wherein the zones of hot combustion gases areidentified by observation or by computer modeling.
 18. The method ofclaim 17 wherein the computer modeling includes flow modeling of highvelocity injectors designed to form nitric oxide reburn zones.
 19. Themethod of claim 12 also comprising injecting reburn fuel at velocitiessufficient to form fuel-rich reburn zones within at least one of thezones of hot combustion gases.